UE4 How do I have an actor be stunned and pause AI in Behavior Tree?

Hi, I am very new to Unreal Engine 4 and Behavior Trees. I’ve set up Blackboard Key called “IstStunned” and I plan to use that as a decorator. If “IsStunned” is true, the AI will be paused.

However, I’m having trouble having the Blackboard Key be changeable. I want to make it so that if the actor is hit by the player, they are stunned. I know how to make the actor take damage on hit already. However, I don’t know how to make the actor set the Blackboard Key “IsStunned” to true when they are hit. I tried doing this in the actor’s blueprint and there doesn’t seem to be a way to set Blackboard Keys. Can someone show me how to do this step by step?